I've been working on a wordpress post loop. I have it set to show 12 items and then show the Next/Previous buttons. Can anyone help?

I also wanted to try to figure out how to add a banner that spans 100% after 6 items have been show.

<?php query_posts('category_name=Portfolio'); ?>
                <?php if ( have_posts() ) : ?>
                  <?php $i = 1; while (have_posts() && $i < 12) : the_post(); ?>
                    <?php if($i == 1) {?><div class="row"> <?php } ?>
            <div class="col col_1_3">
              <!-- <img src="<?php the_post_th// umbnail();?>" alt="<?php // post_title(); ?>" /> -->
              <div class="port-img-border"><?php the_post_thumbnail();?></div>
                      <h6 class="portfolio-title"><?php the_title(); ?></h6>
                      <div class="port-buttons">
                        <?php if(get_post_meta($post->ID, 'Website_URL', true)) { ?>
                          <a class="view" href="<?php echo get_post_meta($post->ID, 'Website_URL', true) ?>" target="_blank"><em data-icon="("></em><span>Link 1</span></a>
                        <?php } ?>
                        <a class="view" href="<?php the_permalink(); ?>"><em data-icon="G"></em> <span>Link 2</span></a>
                      </div>
            </div>
            <?php if($i == 3) { ?></div><? } ?>
          <?php $i++; endwhile; ?>
          <p><?php next_posts_link(); ?></p>
          <p><?php previous_posts_link(); ?></p>
        <?php else : ?>
          <h1>Not Found</h1>
          <p>Silly monkey.</p>
        <?php endif; ?>